The main reason to avoid any sort of mechanical kill is time. Even if you could deploy and retract a puncher in 100 ms, you'd travel 10 inches during that time. The slowest operation in row crops is about 6mph. Same principle applies to microwaves & lasers. All require time over the target. Chemicals are nice b/c they do their work after your machine leaves and its easy to get a solenoid that opens and closes in 5ms.
There are other issues w/ exploding seed head making the problem worse and w/ making reliable complex moving parts in the horrible ag environment.
Microwaves might also be an interesting approach. We should chat.